This is a list of Photoshop filters and plug-ins that I find useful.

theimagingfactory.com Convert To B&W Pro

This is by far the best black & white converter I have found yet. You can play with any color and density filter you can imagine, adjust the color response 6-band EQ however you like (there are presets to match Tri-X and other popular B&W films), and play with the tonal response to get the desired level of contrast. You can even tint the final output if so desired. At $99, it's a little pricey, but it's a must-have for the serious digital B&W enthusiast.

theimagingfactory.com Debarrelizer

Annoyingly, this filter fixes barrel distortion, but not pincushion. But (and this is a big but) it also corrects chromatic aberration very well, which is what I mainly use this filter for. So it's still worth the $39.95, but it would be an even better value for the money if it handled pincushion distortion, too.

theimagingfactory.com Perspective

This filter applies the digital equivalent of a camera movement to the image, digitally tilting and/or rotating the image. Useful for giving a "straight-on" perpective to an image, even if the camera was tilted or angled when the shot was taken. Another $39.95 well-spent.